In a variety of environments, signals must be transmitted between diverse sources and circuitry using those signals, while maintaining electrical (i.e., galvanic) isolation between the sources and the using circuitry. Isolation may be needed, for example, between microcontrollers, on the one hand, and devices or transducers which use microcontroller output signals, on the other hand. Electrical isolation is intended, inter alia, to prevent extraneous transient signals, including common-mode transients, from inadvertently being processed as status or control information, or to protect the equipment from shock hazards or to permit the equipment on each side of an isolation barrier to be operated at a different supply voltage, among other known objectives and uses.
A variety of isolation techniques are known, including the use of optical isolators that convert input electrical signals to light levels or pulses generated by light emitting diodes, and then to receive and convert the light signals back into electrical signals. Isolators also exist which are based upon the use of Hall effect devices, magneto-resistive sensors, capacitive isolators and coil- or transformer-based isolators.
Optical isolators, which are probably the most prevalent, present certain well-known limitations Among other limitations, they require significant space on a card or circuit board, they draw a large current, they do not operate well at high frequencies, and they are very inefficient. They also provide somewhat limited levels of isolation. To achieve greater isolation, optical-electronic isolators have been made with some attempts at providing an electrostatic shield between the optical transmitter and the optical receiver. However, a conductive shield which provides a significant degree of isolation is not sufficiently transparent for use in this application.
In the area of non-optical isolation amplifiers for use in digital signaling environments, U.S. Pat. No. 4,748,419 to Somerville, shows the differentiation of an input data signal to create a pair of differential signals that are each transmitted across high-voltage capacitors to create differentiated spike signals for the differential input pair. Circuitry on the other side of the capacitor barrier has a differential amplifier, a pair of converters for comparing the amplified signal against high and low thresholds, and a set/reset flip-flop to restore the spikes created by the capacitors into a logic signal. In such a capacitively-coupled device, however, during a common mode transient event, the capacitors couple high, common-mode energy into the receiving circuit. As the rate of voltage change increases in that common-mode event, the current injected into the receiver increases. This current potentially can damage the receiving circuit and can trigger a faulty detection. Such capacitively coupled circuitry thus couples signals that should be rejected. The patent also mentions, without elaboration, that a transformer with a short R/L time constant can provide an isolation barrier, but such a differential approach is nonetheless undesirable because any mismatch in the non-magnetic (i.e., capacitive) coupling of the windings would cause a common-mode signal to appear as a difference signal. Transformer cost and size may also be a negative factor, and transformers having cores of magnetic materials such as ferrites become inefficient at high frequencies and are not useful for coupling high-speed digital signals.
Commonly-owned U.S. Pat. No. 5,952,849 shows another logic isolator which avoids use of optical coupling. This logic isolator exhibits high transient immunity for isolating digital logic signals.
A need exists, however, for a less expensive, higher performance digital signal isolator with good dynamic characteristics at high frequencies or speeds.
Moreover, needs exist for logic isolators which provide improved low-cost bidirectional signal transmission capabilities and which can be configured for a variety of signal transmission configurations.
A need further exists for improved signaling schemes for use in isolators, to permit a logic isolator to be based on a single micro-transformer.